As is known in the art, percutaneous transluminal coronary angioplasty or simply balloon angioplasty is a technique in which arteries obstructed by plaque build up are cleared. One problem with this technique however is that abrupt reclosure and gradual refilling of the artery typically occurs in about 17-34% of those cases in which the technique is used.
As is also known, laser radiation has been used in conjunction with balloon angioplasty to deliver power to plaque and weld it in place. The laser radiation technique is primarily a thermal process, dependent on maintaining an elevated plaque temperature level in the range of about 95.degree. to 143.degree. C. One problem with this technique, however, is that it is relatively difficult to distribute the laser energy uniformly around the plaque. Furthermore it is difficult to apply the correct intensity to heat plaque without overheating healthy artery tissue.
Another approach used to deposit power in conductive media is microwave irradiation. Radio-frequency (RF) energy including microwave energy may be used to non-invasively generate heat within tissue volumes. Although there are wide variations in the types of biological tissue, the electrical characteristics of biological tissue may generally be grouped into two classes. The first class includes high-water-content tissue (HWC), including muscle, organ, and blood and blood vessel walls. The second class includes low-water-content tissue (LWC), including fat and bone. Atherosclerosis plaque which collects on the inner walls of blood vessels is generally composed of lipids and calcium particles, and thus may also be considered LWC. The dielectric constants and conductivities of HWC and LWC tissue classes vary directly, although non-linearly, with frequency. Generally, HWC values are several times greater than LWC values.
With such large differences in electrical characteristics between LWC and HWC tissue, radiation may be strongly reflected at tissue boundaries, and power dissipation depends on the angle at which the incident wave impinges the boundary between the LWC and HWC tissue. The tangential electric field component must be continuous across the boundary between the LWC and HWC tissue, but the component of the electric which is normal to the boundary is much smaller in the HWC tissue. Dissipated power P may be computed ##EQU1## in which E corresponds to an electric field intensity and .sigma. corresponds to electrical conductivity. The electrical conductivity is greater in HWC tissue than LWC tissue, thus, the power P deposited by a normally-polarized electric field on the HWC tissue may be significantly less than in the LWC side.
On the other hand, the tangential electric field polarization is continuous across the boundary and will deposit approximately one order of magnitude more power on the HWC side. Conventional microwave-assisted balloon angioplasty devices tend to generate an electric field having an electric field polarization which is primarily tangential to the artery wall. Consequently, conventional microwave balloon angioplasty devices may overheat healthy tissue. Thus it would be desirable to provide an antenna which may be used in balloon angioplasty devices and which does not overheat healthy tissue.